Preservation in microbial mats: mineralization by a talc-like phase of a fish embedded in a microbial sarcophagus

Abstract

Microbial mats have been repeatedly suggested to promote early fossilization of macroorganisms. Yet, experimental simulations of this process remain scarce. Here, we report results of 5 year-long experiments performed onfish carcasses to document the influence of microbial mats on mineral precipitation during early fossilization. Carcasses were initially placed on top of microbial mats. After 2 weeks, fish became coated by the mats forming a compact sarcophagus, which modified the microenvironment close to the corpses. Our results showed that these conditions favored the precipitation of a poorly crystalline silicate phase rich in magnesium. This talc-like mineral phase has been detected in three different locations within the carcasses placed in microbial mats for more than 4 years: (1) within inner tissues, colonized by several bacillary cells; (2) at the surface of bones of the upper face of the corpse buried in the mat; and (3) at the surface of several bones such as the dorsal fin which appeared to be gradually replaced by the Mg-silicate phase. This mineral phase has been previously shown to promote bacteria fossilization. Here we provide first experimental evidence that such Mg-rich phase can also be involved in exceptional preservation of animals.

The Effect Of microbial Mats In The Decay Of Anurans With Implications For Understanding Taphonomic Processes In The Fossil Record

“…The results provided data on the decay sequence, the microenvironmental conditions (pH and DO), and the influence of the mat on soft-tissue durability. The main result was that frogs in microbial mats showed a significant delay in decay, and the body maintained articulation over the years, replicating other results obtained with fish292446. Bodies in control tanks showed fast decomposition, starting with a loss of integrity around day 30.…”

“…Tanks were filled with water from Chiprana until the column had a depth of approximately one centimetre. Previous experiments29 verified that the presence of this limestone layer did not significantly influence the chemical composition of the water column. The Ca, carbonates, sulphate and magnesium concentrations at the end of the experiment were similar to those observed at beginning of the experiment (detailed water analysis can be found in29).…”

“…In this sense, although it is broadly accepted that acidity and anoxia are necessary for calcium phosphate precipitation, these kinds of minerals can also occur under mat conditions according to the mineralogical data5051. The variation in mineralization types seems to provide new and previously unknown evidence, for instance, the appearance of a Mg-rich silicate phase replacing the hydroxyapatite of fish rays and scales29. This amorphous silicate-phase has been recently described as a good carbonate precursor in the course of microbialite formation52.…”

“…Fossilization by microbial mats also includes authigenic mineralization by lithifying (e.g., refs 25, 26, 27, 28). Experiments conducted with fish demonstrate the authigenic formation of a poorly crystalline Mg-silicate phase in relation to the decay of carcasses29.…”

“…Our aim is to decipher the potential for exceptional preservation by exploring the permanence of cadaver integrity and long-term tissue preservation (several years). Previous studies with modern fish have partially reached these objectives242930. In fact, in the case of vertebrates, experimental taphonomic approaches have been frequently conducted with fish (e.g., refs 31 and 32), with a few studies on other groups (e.g., birds ref.…”